1. Field of the Invention
This invention relates to a lumbar support provided in the back of a seat.
2. Description of the Prior Art
An example of a conventional lumbar support provided in the back of a seat is disclosed in the specification of Japanese Patent Publication (KOKOKU) No. 57-19965. The disclosed lumbar support includes a seat back body, a resilient member supported by the seat back body, a lumbar plate one surface of which is abutted against the resilient member in such a manner that the resilient member is embraced by the lumbar plate and the seat back body, and plate moving means for moving the lumbar plate back and forth.
As shown in the sectional view of FIG. 1, the plate moving means comprises a base portion 41 supported by the seat back body; a screw rod 43 supported for free rotation on the base portion 41 and having a male screw provided on its outer peripheral surface; a handle 44 for turning the screw rod 43; a nut member 45 having a centrally formed female screw threadedly engaged with the male screw of the screw rod 43 and a pair of grooves 46 formed in its outer peripheral surface, the nut member 45 being moved longitudinally of the screw rod 43 by turning the latter; a torsion spring 49 having its one end connected to the approximate central portion of a lumbar plate 47 and having its other end fitted into the grooves 46, the base end portion of said other end having a ring-shape pivot portion 50; and a retaining pin 51 passed through the ring-shaped pivot portion 50 and having both ends retained in round holes 52 (only one of which is shown) provided in the base portion 41.
As shown in FIG. 2, the torsion spring 49 is fitted into the drum-shaped grooves 46 in the outer periphery of the nut member 45 with some play in the axial direction of the screw rod 43 so as to allow a change in the relative angle between the torsion spring 49 and the nut member 45. Further, the opposite surfaces of each groove 46 provided in the outer periphery of the nut member 45 is made arcuate in contour relative to a plane perpendicular to the axis of the screw rod 43 to reduce the amount of contact between the torsion spring 49 and the grooves 46 in such a manner that the change in the relative angle between the torsion spring 49 and nut member 45 may take place smoothly. In other words, the torsion spring 49 is provided with play along the axis of the screw rod 43. This play is essential for the purpose of moving the lumbar plate 47 to the desired position.
However, due to the play in the axial direction of the screw rod 43, the amount the handle 44 is turned and the amount the lumbar plate 47 is moved are not in one-to-one correspondence. This means that movement of the lumbar plate 47 exhibits poor reproducibility. Furthermore, since there is the play in the axis of the screw rod 43, the nut member 45 rotates together with the handle 44 for a short period of time when the handle 44 is turned, as shown in FIG. 3, and the nut member 45 does not move in the axial direction of the screw rod 43. Thus, the result again is lack of one-to-one correspondence between rotation of handle 44 and movement of lumbar plate 47.
Since there is no reproducibility between rotation of the handle 44 and movement of the lumbar plate 47, adjusting the position of the lumbar plate 47 is a complicated and troublesome task.